3.1 Thermochemical Conversion Technologies
69
HTL of biomass/wastes is the limitation on particle size and solid concentration of
the feedstock for pumping and possible clogging of the pump and the reactor.
3.1.3.1 Effects of Operating Conditions on HTL
Operating parameters such as reaction temperature, retention time, presence of a
catalyst and water-to-biomass ratio are key variables that affect the yields and characteristics of HTL products. Feedstock type and composition can largely affect the
produced bio-oil characteristics (heteroatom content, type of compounds and functional groups of the oils). Thus, proper characterization and selection of the feedstock
based on the intended downstream application is necessary. As a general trend the
conversion rates for different biomass constituents under HTL conditions are in the
order of lipids > proteins > hemicellulose > cellulose > lignin. HTL of the sludge
feedstock, mainly composed of cellulose and lignin, results in bio-oils containing a
high content of ester, phenolic and nitrogenous compounds. Whereas liquefaction of
algal biomass with high protein content produces bio-crude with a high percentage of
nitrogen compounds, and the oils from swine manure are mostly composed of bulk
carbohydrates, crude lipids and a mixture of phenolic and lipid-derived compounds
[38].
Many studies have shown that using catalysts in HTL of biomass could effectively enhance the liquefaction efficiency by improving biomass conversion, bio-oil
yields and quality (heating values) [43–48]. Different kinds of homogeneous and
heterogeneous catalysts have been tested in the HTL process, and alkali catalysts
demonstrated to be very effective for increasing the bio-oil yields, and the most
common alkali catalysts used are alkali metal compounds such as Na 2 CO 3 , NaOH,
K 2 CO 3 , KOH, LiOH, RbOH and CsOH [2, 48, 50, 51].
A higher temperature normally resulted in enhanced biomass conversion and
bio-oil yields; however, beyond a certain temperature, the oil yields leveled off or
peaked accompanied with an increased yield of solid residues (char) likely due to
the high-temperature cracking of the degradation products and condensation and repolymerization of the reaction intermediates at elevated temperatures [44]. A similar
trend can be observed for the effects of retention time on the bio-oil yield in biomass
HTL [44].
3.1.3.2 Industrial Applications of Hydrothermal Liquefaction
Although the industrial scale HTL process is not yet available worldwide by far, HTL
processes in pilot and demonstration scales have been practiced in several countries,
as shown in Table 3.5. For better economic feasibility of an HTL process, the dry
matter content of the feedstock should be sufficiently high. To overcome this challenge, co-processing of high-water-content (over 90%) wastewater sludge and other
lignocellulosic biomass was patented by the authors and was licensed to the Energy
Research Institute, Jiangxi Academy of Sciences, China, in 2019 for pilot-scale
69
HTL of biomass/wastes is the limitation on particle size and solid concentration of
the feedstock for pumping and possible clogging of the pump and the reactor.
3.1.3.1 Effects of Operating Conditions on HTL
Operating parameters such as reaction temperature, retention time, presence of a
catalyst and water-to-biomass ratio are key variables that affect the yields and characteristics of HTL products. Feedstock type and composition can largely affect the
produced bio-oil characteristics (heteroatom content, type of compounds and functional groups of the oils). Thus, proper characterization and selection of the feedstock
based on the intended downstream application is necessary. As a general trend the
conversion rates for different biomass constituents under HTL conditions are in the
order of lipids > proteins > hemicellulose > cellulose > lignin. HTL of the sludge
feedstock, mainly composed of cellulose and lignin, results in bio-oils containing a
high content of ester, phenolic and nitrogenous compounds. Whereas liquefaction of
algal biomass with high protein content produces bio-crude with a high percentage of
nitrogen compounds, and the oils from swine manure are mostly composed of bulk
carbohydrates, crude lipids and a mixture of phenolic and lipid-derived compounds
[38].
Many studies have shown that using catalysts in HTL of biomass could effectively enhance the liquefaction efficiency by improving biomass conversion, bio-oil
yields and quality (heating values) [43–48]. Different kinds of homogeneous and
heterogeneous catalysts have been tested in the HTL process, and alkali catalysts
demonstrated to be very effective for increasing the bio-oil yields, and the most
common alkali catalysts used are alkali metal compounds such as Na 2 CO 3 , NaOH,
K 2 CO 3 , KOH, LiOH, RbOH and CsOH [2, 48, 50, 51].
A higher temperature normally resulted in enhanced biomass conversion and
bio-oil yields; however, beyond a certain temperature, the oil yields leveled off or
peaked accompanied with an increased yield of solid residues (char) likely due to
the high-temperature cracking of the degradation products and condensation and repolymerization of the reaction intermediates at elevated temperatures [44]. A similar
trend can be observed for the effects of retention time on the bio-oil yield in biomass
HTL [44].
3.1.3.2 Industrial Applications of Hydrothermal Liquefaction
Although the industrial scale HTL process is not yet available worldwide by far, HTL
processes in pilot and demonstration scales have been practiced in several countries,
as shown in Table 3.5. For better economic feasibility of an HTL process, the dry
matter content of the feedstock should be sufficiently high. To overcome this challenge, co-processing of high-water-content (over 90%) wastewater sludge and other
lignocellulosic biomass was patented by the authors and was licensed to the Energy
Research Institute, Jiangxi Academy of Sciences, China, in 2019 for pilot-scale